Exhaust aftertreatment systems are used to receive and treat exhaust gas generated by IC engines. Conventional exhaust gas aftertreatment systems include any of several different components to reduce the levels of harmful exhaust emissions present in exhaust gas. For example, certain exhaust aftertreatment systems for diesel-powered IC engines include a selective catalytic reduction (SCR) catalyst to convert NOx (NO and the NO2 in some fraction) into harmless nitrogen gas (N2) and water vapor (H2O) in the presence of ammonia (NH3).
Many aftertreatment systems include a particulate filter (e.g., a diesel particulate filter [DPF]) positioned upstream of the SCR system. The particulate filter is configured to filter particulate matter (PM), for example soot entrained within the exhaust gas flowing through the aftertreatment system to meet PM emissions regulations. Generally, particulate filters have a very high mass filtration efficiency (99.9%). Particulate filters are generally configured to filter particles of a size smaller than about 100 nm very effectively by diffusion processes and also filter particles of a size larger than about 300 nm are very effectively by impaction and interception. However, there is a known phenomenon sometimes referred to as “the Greenfield Gap,” where particles falling within this size range (i.e., between 100-300 nm) are filtered at a much reduced efficiency. As such, particles that are not filtered by a particulate filter can be thought of as effectively “size selected”.
Exhaust emission regulations require monitoring of the particulate filter to ensure that the particulate matter emissions meet regulation standards. While an amount of particles having a size within the least efficient size range might pass through the particulate filter, the amount is generally sufficiently small such that total PM emissions remain within the emission standards for a properly functioning particulate filter. However, catastrophic failure of the particulate filter, for example a crack or break in the particulate filter, can provide a relatively uninhibited flow path for the PM to flow to through the particulate filter to the downstream aftertreatment components (e.g., the SCR system) and into the environment. In such a scenario, the large particles having a size above the Greenfield gap size range, which are normally intercepted by the particulate filter, also pass through the filter and significantly increase the PM emissions.